Ultrasound based Crack Depth Estimation in Steel-fiber Reinforced Concrete

Abstract

Estimation of the depth of surface-breaking cracks caused by bending in concrete has crucial importance in order to predict the remaining load capacity of a structural member. In practice, ultrasonic tests are the most commonly used non-destructive methods to assess the condition of concrete. However, the commercial ultrasound-based methods focus on the estimation of thickness of the structural element rather than the crack depth. The cracks cause dispersion and attenuation in the propagating waves, and thus by monitoring the changes in these wave characteristics, diagnostic indexes correlated with the crack depth can be defined. This paper explains this approach through the tests performed on seven laboratory-scale steel-fiber reinforced concrete beams (50x10x10 cm3). The beams are loaded under the crack-controlled three-point bending test until a specific crack depth is reached. These beams are then subjected to ultrasonic testing to acquire the propagating surface waves. The recorded signals are analysed by utilizing signal processing techniques, including discrete wavelet transform and frequency-wavenumber analysis in order to extract two diagnostic features, namely, material attenuation coefficient and dispersion index. It is shown that both diagnostic features are able to detect the crack and estimate its depth.

Keywords

• Non-destructive testing
• crack depth estimation
• steel-fiber reinforced concrete
• ultrasonic waves
• dispersion
• attenuation

Ultrasonik Yöntemler ile Çelik-lif Takviyeli Betonda Eğilme Çatlaklarının Derinlik Tayini

Abstract

Betonda eğilme çatlaklarının derinliklerin tespit edilmesi, yapısal elemanların kalan yük taşıma kapasitelerinin tayini için önem taşımakladır. Uygulamada, mevcut tahribatsız ultrasonik test yöntemleri çatlak derinliğinin tespitinden ziyade ya elemanların kalınlıklarının ölçülmesinde ya da mukavemet tayininde kullanılmaktadır. Bir cisim içinde yayılmakta olan ultrasonik dalgalar bir çatlak ile karşılaştıkları zaman dalga karakteristiklerinde değişim gözlemlenir. Bu değişimin takibi ve ölçümü ile çatlak derinliğinin tayini mümkündür. Bu makale, bu yaklaşıma dayanarak geliştirilen tanı metodunu, 7 adet çelik-lifli beton kiriş numune (50x10x10 cm3) üzerinde gerçekleştirilmiş olan ultrasonik testler üzerinden incelemektedir. Kiriş numeneler çatlak-kontrollü üç-noktalı-eğilme testi ile belli bir çatlak derinliğine ulaşılana kadar hasara uğratılmış ve akabinde gerçekleştirilen ultrasonik testler ile dalga yayılımı kaydedilmiştir. Kaydedilen dalga sinyal serileri, ayrık wavelet dönüşüm ve frekans-dalganumarası analizi gibi dijital sinyal işleme teknikleri ile analiz edilerek iki tip tanı indeksi elde edilmiştir. İlk tanı indeksi malzeme sönüm katsayısı α, dalga sönümünü; diğer tanı indeksi ‘dağılım indeksi DI’ ise, dalgaların faz hızındaki dağılımı temsil etmektedir. Her iki tanı indeksinin de çatlak tespiti ve derinlik tahmininde faydalı oldukları görülmüştür.

Keywords

• tahribatsız muayene
• çatlak derinlik ölçümü
• çelik-lif takviyeli beton
• ultrasonik yüzey dalgaları
• dalga sönümü
• dalga dağılımı

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